1. Field of the Invention
This invention relates generally to submersible pump motors, and in particular to the rotor bearings used therein.
2. Discussion of the Related Art
Submersible pumping systems have been employed in the pumping of oil and water from wells for many years. Typically, a submersible pumping system comprises an electric motor, a motor protector, and a pump suspended colinearly in a well casing by tubing or cable. The pump is generally a centrifugal pump which is coupled to the motor. The motor rotates a power transmission shaft that concurrently operates the pump. The motor and motor protector are filled with oil to aid in heat dissipation, to maintain proper internal lubrication of the motor, and to separate the internal components of the motor from surrounding wellbore fluids.
Because these pumping systems are generally disposed within a narrow well casing, the motor, motor protector, and pump are generally long and cylindrically shaped. The motors vary in horsepower depending on the application. Accordingly, the motors of submersible pumping systems can be quite long leading to particular difficulties not encountered in other electric motor applications.
The motors of submersible pumping systems are typically comprised of a stator secured within a tubular housing and a rotor secured to a power transmission shaft that rotates within the stator. The rotor typically is made up of a number of rotor sections, the number of rotor sections depending upon the length and power rating of the motor. Generally, each rotor section is comprised of laminated steel plates or disks secured by copper rods. The rotor sections are spaced apart from each other, and a rotor bearing assembly is located between each rotor section. Each rotor section is keyed to the shaft so that all of the rotor sections rotate as the shaft does.
Each rotor bearing assembly within a rotor section acts to support the shaft and to maintain it in proper axial alignment. A rotor bearing assembly is generally comprised of a sleeve keyed to the shaft so that the sleeve rotates as the shaft does, and a journal or bearing, disposed coaxially around the sleeve. The sleeve and journal are rotatively coupled to one another. The journal is configured to frictionally engage the inner wall of the stator to prevent the bearing from rotating and to maintain proper alignment of the shaft. Thus, a portion of the rotor bearing assembly is rigidly coupled to the shaft but not to the stator.
Due to the high operating temperatures within the well, thermal expansion tends to cause the shaft, rotor, and stator to grow axially. Generally, the rotor and shaft tend to grow axially downwards during high temperature operation. The stator also tends to grow axially downwards, however, to a lesser extent than the rotor and the shaft. Due to these thermal expansion effects, the motor is constructed so that each rotor bearing assembly attached to the motor shaft within a rotor section offers a limited amount of axial mobility. Thus, because each rotor bearing assembly is coupled to the motor shaft, the shaft retains the same limited amount of axial mobility. Axial mobility is limited by thrust washers adjacent to each rotor bearing assembly.
Angular misalignment of the shaft within the motor can occur because the rotor, shaft, and stator are subject to these dimensional changes due to thermal expansion and because of imbalances in the rotating assembly. Misalignment of the shaft during operation opposes the centering, or aligning force of the bearing assembly and causes vibrations within the motor. Excess vibration can lead to premature motor or component failure.
Ideally, the journal will remain stationary while the sleeve, rotor, and shaft are rotating. Previously, rotor bearing assemblies have been used in which the peripheral surface of the journal frictionally engages the inner surface of the stator through metal-to-metal contact, such as via a metallic washer. Such metal-to-metal frictional fit rotor bearing assemblies have a tendency to become loose and then to rotate with the shaft. Rotation of the journal tends to gouge and deface the inner surface of the stator. Once the journal begins to rotate with the shaft, the centering force of the rotor bearing assembly is diminished leading to increasing angular misalignment, vibration, and motor failure. This type of construction is also unsatisfactory because due to thermal expansion of the bearing assembly during motor operation, the journal may tightly engage the stator wall which can cause angular misalignment of the shaft and thus excessive thrust loads onto the thrust bearing surfaces adjacent to the rotor bearing assembly.
Various types of elastomeric materials have been interposed between the journal and the inner surface of the stator in an effort to hold the journal stationary while also allowing the necessary axial mobility of the journal vis-a-vis the inner surface of the stator. These elastomeric materials have been in the form of O-rings or other similarly configured and annularly disposed means. While initially operating satisfactorily, the elastomeric materials tend to lose their elastic memory due to the effects of thermal expansion and contraction during periods of operation followed by inactivity. With the loss of the elastic memory of the elastomeric material, the bearing becomes loose and rotatable. Once the journal begins to rotate with the shaft, the centering force of the rotor bearing assembly is diminished leading to increasing angular misalignment, vibration, and motor failure. Such elastomeric elements also render the motor more difficult to assemble, and may be easily damaged by adjacent metal parts during assembly of the rotating assembly and stator.